Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Inkjet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in both cases. Commonly used carrier media include water, mixtures of water and organic co-solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, etc.
In traditional dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness and poor smear resistance. These problems can be minimized by replacing the dyes used in ink formulations with insoluble pigments. In general, pigments are superior to dyes with respect to waterfastness, lightfastness, and stability towards pollutants in the air. However, the pigment inks tend to be unstable and settle out from the liquid vehicle over a long storage time. Pigment inks also have a tendency to clog the orifices of the printhead resulting in deterioration of print quality.
Accordingly, there is a need for inks having advantages of both dye-based inks and pigment based inks that have good lightfastness, stability towards pollutants, waterfastness and reliability in the printhead.
Polymeric colorants, such as polymeric dyes are known. In comparison with ordinary colorants, polymeric colorants offer the advantage of allowing a range of physical properties. Their solubility, absorption, migration, and viscosity are tunable and they do not sublime, are non-abrasive, and generally have low toxicity.
U.S. Pat. No. 4,644,708 discloses an ink composition comprising water and a polymeric dye comprised of the reaction product of a water-soluble polymer with a reactive dye. However, there is a problem with this dye in that its viscosity is high, resulting in poor firability from a thermal ink jet printhead.
U.S. Pat. No. 5,098,475 discloses the preparation of dendrimeric dyes by reacting a dendrimer with a dye and using the dendrimeric dye in ink formulations. However, there is a problem with these dendrimeric dyes in that dendrimers are typically prepared through lengthy multi-step syntheses, and their availability is limited to a small group of functional monomers.
It is an object of this invention to provide an ink jet ink composition that, when printed, provides an image which has improved waterfastness, lightfastness, and stability towards pollutants such as ozone. It is another object of the present invention to provide an ink jet ink composition that has improved firability through an ink jet printhead.